Blood culture results obtained in a single tertiary neonatal intensive care unit are reviewed. In 4416 admissions occurring over 6 y we identified 206 positive cultures (4.7/100 admissions) growing 234 bacterial and fungal isolates in 182 infants. Very early and early onset positive cultures comprised 17% and 22% each. Gram-positive bacteria dominated in very early (61%), early (91%) and late onset (78%) cultures with coagulase-negative staphylococci (CONS) as the most frequent isolate in all groups (22%, 46% and 55%, respectively). The 3 most frequent isolates following CONS were in very early onset cultures Escherichia coli (19%), anaerobic bacteria (17%) and group B streptococci (GBS) (14%), in early onset cultures Staphylococcus aureus (28%), Enterococci (7%), E. coli (6%) and Viridans streptococci (6%) and in late onset cultures S. aureus (15%), Candida species (8%) and E. coli (5%). Infants < or = 999 g birthweight, representing 6% of the admissions, contracted 37% of the positive blood cultures and nearly half (44%) of the CONS isolates. In these patients, a significant increase (p < 0.001) in the number of positive cultures/100 admissions and in the proportion of positive cultures in conjunction with an intravascular catheter were seen (p < 0.001). An intravascular catheter was more often present when CONS were isolated as compared to other organisms (p < 0.05). 23 positive cultures (11.2%), most frequently E. coli, were associated with a fatal outcome. Our microbiological pattern is dominated by a gram-positive flora, which is in agreement with recent European and North American reports, but differs from earlier Scandinavian studies in the proportion of CONS and GBS reported.